Recently there has been a growing interest in the so called beyond 3rd generation (B3G) wireless systems. There seems to be a common understanding that these systems will be composed of heterogeneous technologies both on the terminal side (mobile phones, laptops, PDA's, etc.) and the network side (fixed and radio access technologies, transmission systems, etc.) [1], [2]. From the end-user's perspective, these systems promise to deliver ubiquitous connectivity [3] and a wide range of high-quality services [4], [5].
Ubiquitous service provision involves scenarios where connectivity is provided to passengers traveling on some vehicle, such as a bus, train, boat or an aircraft [3], [6]. In fact, such local moving or vehicular network scenarios have recently been identified as an important standardization area by the Internet Engineering Task Force [7].
In a moving network scenario, in which the vehicle uses a wireless (satellite, cellular or WLAN) link to be connected to a fixed access point (base station); the available bandwidth between the local network in the vehicle and some stationary access point or access router can vary as the vehicle moves between access points. In general, these access points do not have to belong to the same radio access technology (RAT). For instance, when a train arrives at a station, it may provide connectivity through a large-capacity wireless local area network (WLAN). When leaving the station it may switch to cellular access such as a 3G or UMTS network. Also, while within the same RAT, available bandwidth may fluctuate due to mobility or changes in the radio conditions (fading, shadowing, etc). Due to the variations in the available bandwidth for the vehicular network, individual users, who share the total available bandwidth, will experience a variation in the bandwidth available for each user.
In a multi-access scenario with vehicular networks, all traffic coming from the users connected to a wireless gateway can be aggregated in a few shared bearers having high bandwidth. Each such bearer carries a specific type of traffic, i.e. data traffic of similar type from several users is multiplexed into the same aggregated or shared bearer.
When the vehicle enters a cell or a new access with different available bandwidth, an adaptation occurs, e.g. one or more shared radio channels (i.e. bearers) between the vehicular network and an available access router in another network are torn down or preferably their respective bandwidth is modified up or down to a suitable level. In a straight forward procedure, one or more users whose traffic prior to the bandwidth change is multiplexed into the shared bearer are dropped.